This invention relates to a means for providing enhanced oxidation stability to synthetic ester-type lubricating oil formulations containing conventional arylamine antioxidants. More particularly, this invention is directed to improved synthetic ester-based lubricating oil formulations containing arylamine antioxidants in combination with certain N-acyl-O-carbamylhydroxylamines which materially improve the high temperature oxidation stability of such formulations.
Synthetic ester-based oils are well known as base fluids for a variety of applications in the lubricant and hydraulic fluid field. In recent years, such synthetic oils have augmented or replaced petroleum oils in many uses in this field, especially those in which severe service conditions are encountered, e.g., high temperature and/or long drain period applications. While these synthetic ester-based fluids possess a number of properties -- e.g., very high viscosity indexes, high flash points, low volatility, etc. -- which have proven quite valuable in extending the operating range of lubricants and hydraulic fluids formulated therefrom, their ultimate utility remains limited by their high susceptability to oxidative degradation.
In the past, a number of efforts have been made to overcome, or at least minimize, the aforementioned tendency for synthetic ester-based fluids to undergo oxidative degradation under extreme operating conditions. A major effort in this regard has been the incorporation of additives such as conventional arylamine oxidation inhibitors, e.g., diphenylamine and phenothiazine, into the synthetic ester-based formulations. Although many of these arylamine oxidation inhibitors provide some measure of protection against oxidation, their deficiencies under severe operating conditions have been recognized and, as a result, it has been proposed to augment the arylamine antioxidant activity with supplemental agents such as antioxidant activators or auxiliary oxidation inhibitors. For example, in U.S. Pat. No. 3,347,791, it is disclosed that the resistance to oxidation provided by arylamine oxidation inhibitors in synthetic ester-based formulations can be enhanced by addition of an antioxidant activator comprising at least one alkali metal salt selected from the group consisting of alkali metal salts of carboxylic acids and alkali metal salts of hydroxy aromatic compounds. While these alkali metal antioxidant activators measurably increase the oxidation resistance of the arylamine-containing synthetic ester-based formulation, they present other problems relating to solubility in the base formulation and corrosivity to metal parts.
Accordingly, from the foregoing it can be seen that a continuing need exists for the development of additives which will enhance or supplement the antioxidant activity of conventional arylamine oxidation inhibitors in synthetic ester-based formulations, while, at the same, being compatible with the base formulation itself and its ultimate use application.